Many historic and architecturally significant buildings have plaster ceilings, walls, and ornamental features. In many of these buildings, over time, the plaster becomes structurally compromised as the mechanical keys that physically hold the plaster onto the lath break. In some cases, the plaster begins to pull away from the lath and starts to sag or fall. In other types of buildings, ceilings may be constructed from plaster panels containing organic plant-based fibers for structural reinforcement. The fibrous plaster panels may be secured to structural framing members solely with plaster-soaked fibers bonded to the backside of the panels and looped over the framing members. Over time, with building movement and water infiltration, the plaster degrades and the organic fibers may become brittle and break, particular if exposed to moisture.
Conservation and restoration work aims to preserve plaster ceilings that are in danger of falling. Historically, one technique for reattaching plaster has been to apply a fast-drying liquid slurry layer of wire or mesh reinforced plaster along the upper side (attic side) of the lath (if it is accessible) so as to bond to the remaining keys and the stubs of the broken keys. Sometimes, a bonding agent is applied to the upper side of the lath and plaster. However, this added weight may be a concern in some cases, and the bond line between the old plaster and the new plaster can be a weak point. Once the material has been applied, no further inspection of the plaster can occur because the bond line is obscured by the newly-applied material.
Another technique that has been used is the injection of a formulation of acrylic-resin-based adhesive into the space between the plaster and the lath. The adhesive may be injected from the backside or, if inaccessible, through holes drilled into the front of the plaster, although the latter approach defaces and damages the plaster when small injection holes are drilled in its visible front surface. This is unsuitable for plaster decorated with valuable paintings or wall coverings. The adhesive fills the gaps between the upper surface of the plaster and the laths and makes a new bonded connection at each location where the adhesive has been injected. An example of adhesive grout for this type of work is described in U.S. patent application Ser. No. 13/749,375, filed Jan. 24, 2013, and owned in common herewith, the contents of which are hereby incorporated by reference.
The injection of grout can be a difficult and expensive process. It may require precision application, which is not easy to achieve in the case where it is being injected through holes in the surface. A full scaffolding installation may be necessary to carry out these types of repairs. Alternatively, full access to the upper side of the ceiling is required. In such a case, significant cleaning of the accumulated debris and dust is necessary before treatment can occur.
These techniques all require access to the structural back of the ceiling or the drilling of holes into the face of the plaster to inject material that, hopefully, bonds to the backside of the plaster. In some situations, the cost and difficulty of this type of rehabilitation cannot be justified, which leads building owners to consider other options. One of the other options is to install a new material over the face of the plaster ceiling. In some cases, this involves laminating drywall (e.g. gypsum board or other such material) over the face of the old failing plaster ceiling. Drywall ceilings installed directly over plaster often exhibit visible joints and sagging in a short period of time, or require the installation of strapping or furring lumber across the face of the plaster, and then the attachment of the drywall to the strapping. Either approach involves the addition of significant weight and, often, destructive drilling into the plaster.
Another option for covering the face of the existing plaster ceiling is to attach wire, wood or plastic mesh or lath to the plaster ceiling surface and then apply a new plaster coat (often multiple coats). The plaster may be conventional or synthetic.
Drywall installation or new plaster coats also usually involve full scaffolding and a great degree of debris and dust. Both add significant weight to the structure.
It would be advantageous to avoid adding significant weight while still structurally reinforcing historic plaster.